Crankwalk Sources

JET's 1G engine had a major crankwalk issue last summer. It got to be so bad the block and crank were destroyed. It is not common for a 1G engine to develop crankwalk, especially the pre-92.5 engines. The 7-bolt engines with manual transmissions have a relatively high occurence of crankwalk, though. 7-bolt engines with automatic trannies have almost zero cases of crankwalk (I have not heard of one).

Crankwalk is the result of thrust bearing wear in the engine. The thrust bearing (a.k.a. the "center bearing") keeps the crank centered longitudinally so it can't move in and out over time. This is necessary to keep the forces presented by the rods on the crank within tolerance to prevent the crank from breaking.

Automatic trannies present virtually no end force on the crank so the thrust bearing has little to do to keep the crank in place. However, the force used to disengage the clutch in a car with a manual tranny also pushes the crank against the thrust bearing. AN ACT 2600, for example, presents as much as 700 pounds of force at the thrust bearing (stock clutches present about 400 pounds). The increased force ought to prematurely destroy the thrust bearing but the superior oiling system in the early 1G engines seems to stave off the inevitable, though it has no right to!

A thrust bearing will fail if the oil film between it and the crank fails. There is to be no metal-to-metal contact there. How an oil film breaks down is beyond the scope of this post, but suffice it to say that oil that is too thick or too thin for the application can result in thrust bearing failure. Thick oil will shear and leave air gaps where oil is supposed to be. This is especially true in engines that rev beyond their design limits. Added disengaement force from stiff aftermarket clutches only exacerbates the problem. Air gaps result in reduced load braing surface area which allows the two surfaces to move closer together which increases oil shear and the creation of even larger air gaps... runway failure!

Another ugly scenario has been discovered as a source for crankwaalk, although it often ruins the transmission first. On many modded DSMs the tranny has been removed many times. It happens that there are two guides that align the tranny with the block. If one or both of these guides is missing, the tranny can quickly become misalinged with the crank. The result is that the tranny input shaft is caused to wiggle around which leads to loosening of the tranny bolts. When this happens the misaligned tranny creates a continuous load on the thrust bearing.

The guides sometimes just fall out when the engine block is being handled. Sometimes they stay with the tranny, too. I have seen this several times and the results both times ruined the tranny input shaft bearing. I suspect this is the cause of JET's regrettable crankwalk, the subject of much debate and ridicule.

Anyway, before the implemenation of modified automatic trannies in drag cars with big block motors, it was common to have to replace the thrust bearings in those motors after just ten times down the track!

Rick